This invention relates to a furnace configuration for high temperature processes which may require high thermal energies such as melting and/or chemical reaction. The invention also relates to a method for the preparation of a product by heating a charge of reactants to a high temperature.
Many high temperature processes are not suited to a scale-up in production from laboratory bench to industrial scale because of their heat transfer requirements and in such cases scale up results in a marked deterioration in furnace performance.
Processes which are limited in their scale-up because of heat transfer requirements are generally those in which the charged material or products formed have a high thermal resistance, undergo endothermic reactions, one component of the reaction is a significantly volatile species at the temperature of the reaction. Upon scale-up, such heat transfer limited processes suffer from the problems of greatly increased reaction times, increased specific energy consumption or incomplete reaction leading to non-uniform product.
Examples of reactions which exhibit such properties are the carbothermic reduction of transition metal or rare earth oxides to transition metal or rare earth borides, carbides and nitrides, production of silicon carbide and boron carbide, silicon smelting, remelting of oxidised metal fines, and ferroalloy production.
Furnaces usually used for production of transition metal borides, carbides and nitrides and similar refractory materials include arc furnaces, resistance furnaces, rotary furnaces, pusher furnaces, induction heated crucible or induction heated shaft furnaces.
Rotary kilns, and pusher furnaces suffer from a low occupancy of charge in the hot zone, so for a given scale, the surface area and heat losses are high. Rotary kilns which can operate under reducing conditions are expensive to engineer for high temperature operation and are limited in their ability to be scaled up.
Induction heated crucibles and shafts suffer from rapid loss of thermal efficiency and long reaction times as soon as the scale exceeds about 3 kg batch.
Various attempts have been made to improve the performance and scale-up of processes limited by heat transfer requirements.
A) Choosing a reactor design which reduces the heat transfer distance.
Methods to reduce the heat transfer distance include the use of multiple small reactors or crucibles in a pusher furnace, the use of a long slender reactor in which one or two dimensions are small, and tumbling the charge as in a rotary kiln. These attempts suffer from the disadvantages of increased surface area to volume ratio and hence high heat losses, and an increase in the size of the furnace hot zone with no corresponding increase in production capacity or efficiency.
B) Driving the heat transfer with high temperature source such as an arc or a plasma.
Arc and plasma furnaces have been designed to place a high intensity heat source at the core of the furnace, and temperatures in the zone of the arc at all stages of reaction are in excess of 2000.degree.-2500.degree. C. Whilst high heat transfer rates can be achieved with the high arc temperature, any components which are volatile may consequentially be lost to the gas phase before they have an opportunity to react to completion. In addition, such furnaces are restricted in their ability to achieve reaction in a narrow temperature range which may be regarded as optimum from a thermodynamic/processing point of view and arc furnaces usually produce a sintered product.
Use of three electrodes enables some reduction of the heat transfer distance, in the horizontal plane, but there is little scope to increase the reaction zone in the vertical direction as this greatly increases heat transfer distances.
Arc furnaces usually use for example, the staffing materials TiO.sub.2 /B.sub.4 C/C for synthesis of TiB.sub.2 because the boron is present in a less volatile form than B.sub.2 O.sub.3 at reaction temperature, and the bulk density of the charge is higher. The high temperature of the arc can then be used to thermally drive the endothermic reaction. The synthesis of B.sub.4 C also made in an arc furnace, however suffers from the same constraints and costs of B.sub.2 O.sub.3 volatility.
B.sub.2 O.sub.3 can be used in the arc furnace charge as a boron source but losses of B.sub.2 O.sub.3 to the gas phase are high and consequently B.sub.2 O.sub.3 is the major cost in raw materials.
C) Using a heat source which generates heat within the charge.
Microwave and resistance heating, and reaction synthesis techniques would at first glance, appear to be the ideal solution to heat transfer limited processes.
Microwave techniques can be used to achieve through heating and high temperatures. However, the containment of microwave energy, the measurement of temperature, the selection of microwave transparent refractories, the low efficiency of conversion of electrical energy to microwaves (50-70%), and the limitation of magnetron sizes to 10-40 KW each, all represent significant problems in engineering microwave based processes. A more fundamental limitation which may occur in a process which undergoes chemical reaction is the uneven power distribution between reactants and products which can lead to thermal runaway in the product.
Resistance techniques have traditionally been used in the Atcheson type furnace, but the technique suffers from hot spots and the cost of high current engineering.
The use of self-propagating high temperature synthesis techniques (SHS) has attracted a lot of attention for synthesis of refractory hard materials, because reactions are chosen so that the reaction is exothermic, instead of strongly endothermic. The method however transfers the problems and costs of endothermic processes to the synthesis of the reactants used. For example Ti and B metal powders can be used to synthesise TiB.sub.2 but the costs of the powders are high. This is justified only when
a) there is no other viable method, PA1 b) there are advantages in obtaining high value-added sintered or fused product, or PA1 c) convenience for small quantities. PA1 an outer shell which enables control of gaseous atmosphere inside the furnace during operation, PA1 an induction means adjacent the outer shell, PA1 a thermal insulating layer insulating the induction means, PA1 a charge receiving space within the insulating layer for receiving the charge, PA1 the charge being placed in the charge receiving space such that the charge occupies a volume defined between an inner limit of the charge and an outer limit of the charge, the inner limit of the charge being defined as a part of the charge which is farthest from said insulating layer, PA1 induction susceptor means positioned within the charge receiving space, said induction susceptor means having a coupling portion that provides a continuous conductive path for induced current within said susceptor means, PA1 the induction susceptor means being positioned within the charge in the charge receiving space such that a cross-sectional center of area of the susceptor means in a radial direction is within the range of about 30%-90% of a radial distance from the inner limit of the charge to the outer limit of the charge, PA1 wherein a progressing zone front for substantially completely reacted product emanates away from the induction susceptor means and the charge located in a volume of the charge receiving space defined between the inner limit of the charge and an inner part of the induction susceptor means has substantially reacted when the progressing zone front reaches and external limit of the charge. PA1 --providing a furnace configuration comprising an outer shell which enables control of gaseous atmosphere inside the furnace during operation, an induction means adjacent the outer shell, a thermal insulating layer insulating the induction means, a charge receiving space within the insulating layer for receiving the charge, the charge being placed in the charge receiving space such that the charge occupies a volume defined between an inner limit of the charge and an outer limit of the charge, the inner limit of the charge being defined as a part of the charge which is farthest from the insulating layer, induction susceptor means positioned within the charge receiving space, the induction susceptor means having a coupling portion that provides a continuous conductive path for induced current within the susceptor means, PA1 --energising the induction means to thereby heat the susceptor means and to cause the charge located adjacent the susceptor means to increase in temperature and to undergo the reaction to produce the product and to form a progressing zone front for substantially completely-reacted product, wherein the progressing zone front emanates away from the susceptor means as time elapses, PA1 --wherein the susceptor means is positioned within the charge in the charge receiving space such that a cross-sectional center of area of the susceptor means in a radial direction is within about 30-90% of a radial distance from the inner limit of the charge to the outer limit of the charge and the charge of reactants located between the inner limit of the charge and an inner part of said susceptor means has substantially reacted when the progressing zone front reaches an external limit of the charge.
In another variation, the reductant for SHS may be Al or Mg powder, which in addition to the above mentioned limitations, leaves an oxide in the product for later separation.
The price of transition metal or rare earth borides on the world market is high because a furnace configuration which is capable of operating at a reasonable thermal efficiency on a large scale and produce quality product with low boron loss has not previously been used.